Guided wave testing is a non-destructive testing method used to inspect pipework for discontinuities in the wall of a pipe, such as corrosion-type defects and cracks. Reference is made to WO 96 12951 A which describes an apparatus and method for inspecting elongate members and which is incorporated herein by reference.
As with most non-destructive testing methods, a guided wave system typically requires calibration so that the received signal amplitudes can be related accurately to the reflection coefficient of the discontinuity. The reflection coefficient is, among other variables such as guided wave wavelength and mode, related to the shape and dimensions of the discontinuity. The reflection coefficient is therefore regularly used to classify defects in terms of severity with respect to the mechanical integrity of the pipework. In order to size defects or other changes in pipe wall thickness, the reflection coefficient is typically converted, using appropriate conversion functions, into an equivalent pipe wall cross-section change. For example, for a circumferentially-oriented, through-wall thickness notch, when using torsional- or longitudinal-type guided wave modes, the reflection coefficient tends to be linearly related to the circumferential extent of the defect and, therefore, also linearly related to the change in pipe wall cross-section caused by the notch. Reference is made to “Defect detection in pipes using guided waves” by M. J. S. Lowe, D. N. Alleyne, and P. Cawley, Ultrasonics, volume 36, pages 147-154 (1998) which is incorporated herein by reference. The reflection coefficient may also be a function of defect depth and axial length. The reflection coefficient may also be a function of the overall shape of the defect. Thus, the conversion function can take different forms.
The methods of calibration currently available tend either to lack accuracy or to make assumptions which cannot be safely assumed to be true.
The most common way of calibrating the size of guided wave reflections is to observe the reflections from girth welds. A typical girth weld reflects approximately 10 to 20% of the guided wave energy, and other indications can be sized relative to the size of the weld reflection. However, the size of the reflection from a girth weld is influenced by the dimensions of girth weld reinforcement, root penetration, width of the girth weld and pipe wall thickness. Weld reinforcement dimensions are not standardised and the dimensions of the weld reinforcement, the root and the width of the weld can vary significantly even within the same pipeline, resulting in a calibration error when assuming a certain size of reflection.
The direct measurement of the weld reinforcement and width using a specialised gauge or measurement of the pipe circumference at the weld location can improve the calibration using girth weld reflections described above. It can be used to calculate the expected size of the reflection, assuming that the girth weld root has a negligible effect on the size of the reflection. However, the dimensions of the weld can vary significantly even within the same girth weld, particularly reinforcement and root penetration, and can therefore lead to significant errors in the calibration. This method of calibration does not take into account the effect of the girth weld root as it cannot be measured.
Furthermore, the use of girth welds for calibration assumes a non-defective weld. If the girth weld is defective in a way which influences the size of the reflection, then any calibration which assumes a certain size of reflection or which involves directly measuring the girth weld reinforcement and width can lead to a significant error in sizing. An example of an in-service defect in a girth weld which can lead such a calibration error is weld root corrosion. An example of a manufacturing defect is insufficient root-penetration.
Calibration using girth welds cannot be used when there are no girth welds within the range of the guided wave test, when they are not accessible for direct measurement or when the size of the reflections from the girth welds is below the detection threshold of the guided wave system, for example when weld reinforcement has been deliberately removed.
A further calibration method involves measurement of the transmitted signal from one transducer ring to a second transducer ring, the second being installed some distance apart from the first. However, the measurement of a transmitted signal from one transducer ring to the second depends on the coupling strength of the two transducer rings on the pipe and therefore can lead to errors in the calibration if the coupling strengths are not equal. Also, this method requires the use of a second transducer ring and so increases equipment requirements. Moreover, this method may also not be practical where there is limited access for the placing transducer rings on the pipe.
US 2004 0216512 A describes a calibration method based on the change in the size of guided wave amplitudes observed when a clamp is placed on a pipe between a discontinuity and the transducer ring. This method requires the use of clamps and assumes that the clamp causes an observable reduction in the size of the reflection from the discontinuity. The use of the clamps increases inspection time and assumes that an appropriate location is available for placing a clamp. Furthermore, when a pipe is coated, the change in observed reflection amplitudes due to the clamp is unlikely to be significant enough for the calibration to be accurate. Thus, removal of the coating would be required.
Referring to FIGS. 1a and 1b, the importance of correctly sizing a defect or other type of reflector will be explained.
FIG. 1a shows the result of a guided wave test, using 36 kHz torsional mode guided wave, of a test pipe (not shown) having an outer diameter of 219.1 mm, a nominal wall thickness of 8.2 mm, two welds, a defect in the positive direction and a square cut end in the negative direction. The square cut end reflects 100% of the guided wave energy. It provides a reference reflector for a 0 dB distance amplitude correction (DAC) curve, i.e. reflection coefficient equals 1.
Adjusting the weld DAC curves to the peak amplitudes of the welds, assuming the weld reflects −16 dB (i.e. a reflection coefficient of approximately 0.16), which is a typical value when using the torsional mode, the defect reflection coefficient is estimated to be approximately 0.09. However, it can be seen that the 0 dB DAC curve does not approximate the reflection from the cut end.
Referring to FIG. 1b, adjusting the 0 dB DAC curve to the cut end, the weld reflection coefficient is estimated to be 0.11 and the defect reflection coefficient approximately 0.06.
This means that the defect size was, based on the assumption that the weld reflects −16 dB of the guided wave energy, overestimated by 50%.
Overestimating the size of defects can lead to costly false calls and decreased reliability of inspection. Equally, the size of a defect can be underestimated when the weld reinforcement is larger than expected or the wall thickness is thinner than assumed, leading to missed calls and again decreasing the reliability of the inspection.